Wow, I haven't seen an MFM cable in years. You really do not want to use that. MFM drives are ancient, slow, big, pieces of {bleep}. Any one you find is going to be very old now and I wouldn't trust any unreplaceable data on one. If you find one cheap/free, maybe use it as a temporary thing, but long term you should find a SCSI or IDE controller.
The "AGA card" (god, I wish there were AGA add-ins for A2000s) is like other people said, a flicker fixer. Those are quite cool, as you can use a PC monitor and use interlace modes without getting a headache.
The RAM card does max out your Zorro bus, so, be careful about other cards you use. I got bit in the ass by that recently myself.
